Time Moves Slower Near Massive Objects

Albert Einstein's theory of general relativity fundamentally changed our understanding of gravity, space, and time. It describes gravity not as a force, but as a consequence of the warping of spacetime by massive objects. Imagine spacetime as a stretched fabric; a heavy object like a planet creates a dip or curve in it. This curvature dictates how everything, including light and time, moves. The stronger the gravitational pull (the deeper the curve), the more slowly time passes. This means that for an observer on the surface of the Earth, time is actually passing slightly slower than for someone in orbit high above.

This remarkable effect is not just theoretical. In 1971, the Hafele-Keating experiment provided direct proof by flying ultra-precise atomic clocks on commercial airliners around the world. When compared to a stationary reference clock on the ground, the clocks that flew at high altitudes, where Earth's gravitational pull is weaker, had run faster. The measured time difference was incredibly small, measured in nanoseconds, but it was consistent with Einstein's predictions. These experiments confirmed that your physical location in a gravitational field directly impacts the rate at which you experience time.

The consequences of this phenomenon are critical for modern technology. The Global Positioning System (GPS) relies on a network (Review) of satellites, each with a highly accurate atomic clock. These satellites are in a region of weaker gravity than we are on the ground, causing their clocks to run faster by about 45 microseconds per day. While their high orbital speed slows the clocks down slightly, the net effect is a gain of about 38 microseconds daily. Without constantly correcting for this relativistic effect, GPS navigation errors would accumulate at a rate of roughly 10 kilometers each day, making the system essentially useless.